This invention relates to a system and process for the processing of the spent black liquor from a kraft pulping process to recover the chemicals and produce fresh pulping liquor. More specifically, the system and process of this invention relate to an integrated system for the warm-up of the gasifier during start-up and for purging the gasifier in emergencies.
The kraft pulping process employs an alkaline pulping liquor, known as white liquor, to react with the lignins in the wood and free the fibrous portions. Following a series of filtering and washing steps, the fibrous portion is separated as raw pulp and the remaining spent cooking liquor, which is dark in color, is known as weak black liquor. This liquor, which is approximately 85% water, is then subjected to a series of various types of evaporation to produce strong black liquor with solids content greater than 50%. The strong black liquor is then ready for the chemical recover phase.
The typical prior art process for treating black liquor to recover chemicals employs what is commonly referred to as a chemical recovery furnace. In these furnaces, which are operated as boilers for the generation of steam, the strong black liquor is fired to burn the organic content and to form a smelt composed primarily of sodium sulfide and sodium carbonate. This smelt is drained from the smelt bed in the bottom of the furnace, dissolved in water to form green liquor and then causticized to form the white pulping liquor containing sodium sulfide and sodium hydroxide.
One of the problems with these typical chemical recovery furnaces has to do with the fact that there is a very hot pool of smelt in the bottom of the furnace and the fact that the furnace is lined with waterwall tubes. If there is a rupture in a waterwall tube and water is leaked onto the smelt bed, there is the potential for a violent explosion which produces high pressures and which can actually blow the furnace apart. It can be seen that systems and processes which avoid this problem would be very beneficial to paper companies.
U.S. Pat. No. 5,284,550 entitled "Black Liquor Gasification Process Operating At Low Pressure Using A Circulating Fluidized Bed," which issued Feb. 8, 1994 and U.S. Pat. No. 5,425,850 entitled "CFB Black Liquor Gasification System Operating At Low Pressures," which issued Jun. 20, 1995 and which are both assigned to the same assignee as the present application, describe and claim one such system and process for replacing a chemical recovery furnace. These patents also discuss as background information, a number of other patents and publications which have attempted in one way or another to solve chemical recovery furnace problems. Referring to the subject matter of U.S. Pat. Nos. 5,284,550 and 5,425,850, they basically involve the replacement of the chemical recovery furnace with a black liquor gasification system using an atmospheric pressure circulating fluidized bed reactor arrangement including the arrangement for processing the gases and solids which are produced to generate fresh cooking liquor. The present invention constitutes a modification for use with that system and process so it will be more fully described hereinafter. However, these two U.S. Pat. Nos. 5,284,550 and 5,425,850 are incorporated herein by reference.